Oxazoline is a five-membered heterocyclic compound containing N and O atoms, wherein the atom of N can coordinate with metal ions as a donor atom. Further, oxazoline can be used as substrate to synthesize various chiral compounds with great success, which makes the chemists believe that oxazoline represents a good outlook in asymmetric catalysis as a ligand. During the past 30 years, a large numbers of chiral oxazoline ligands have been developed, especially those with various chiral side chains. Among others, axial chiral side chain has been widely used in the ligand due to its unique rigid structure. In order to obtain an axial chiral ligand with single configuration, the prior art mainly applies a resolution method or the like, which results in a great extent of waste in resources.
After searching in the prior art, Professor Imai, et. al. (J. Org. Chem., 65, 3326-3333, “Novel Chiral Bisoxazoline Ligands with a Biphenyl Backbone: Preparation, Complexation, and Application in Asymmetric Catalytic Reactions”) reported that an axis unstable ligand 1 adopts two interchanging forms of (S,aS,S)-1 and (S,aR,S)-1 in a solution, while only the (S,aS,S)-1 chelates with a metal salt ion to form an axial chiral ligand compound (S,aS,S)-2, as confirmed through chelating with a metal salt ion. Such an axial chiral ligand compound can be easily synthesized without complicated resolution process. In addition, (S,aS,S)-2 obtains certain enantioselectivity during the asymmetric cyclopropylation reaction that involves the catalysis of olefin. Its disadvantage lies in that: such a ligand is still an axial chiral ligand which is unstable in the axial chirality, and thus it does not break through the design concept of an axial chiral ligand.
